1. Technical Field
The present invention relates to logic circuits. In particular, it relates to secondary power circuits for logic which controls thermal or high voltage devices such as thermal printers.
2. Background Art
The development of computers and computerized printing has allowed many new useful applications to become both inexpensive and commonly available. Typical applications include the use of printers to print tickets on demand for theater performances, receipts for store purchases, etc.
Thermal printers not only provide a low cost solution to on demand printing, but they also provide flexibility in the print content. For example, in addition to alphanumeric data which can be printed in a variety of fonts, data such as bar codes and other symbols can also be printed on the ticket with the alphanumeric data.
To print images, high voltage devices such as thermal printer print heads heat a small point on the paper to create a darkened spot (i.e., pixel). An important factor in any printer is print quality. To increase the dots per inch (DPI) print density, and thereby provide increased image quality, the development of thermal print heads has led to progressively smaller heating elements which in turn allows increased print density. Of course, reducing the size of the heating elements also makes them more delicate and susceptible to damage. Likewise, the same can be said for dot matrix printers which have achieved improved print density through ever smaller and more delicate impact wires.
A second factor which has been an important concern in the development of printers is speed. In the case of thermal printers, improvements in overall print throughput are directly limited by the speed at which the heating elements in the print head heat up or cool down. Therefore, a principle area of improvement in thermal printer speed has been the development of heating element materials which have a high heating/cooling rate. By providing the capacity to generate heat quickly and to dissipate heat quickly, paper can be moved through the printer at a faster rate without degrading print quality.
The simultaneous development of these two factors has resulted in high speed, high quality printers which can be constructed as a compact printer for local use by mainframe computer systems, personal computers, and other business machines such as cash registers.
In addition to the traditional uses discussed above, thermal printers have also found extensive use in other fields, such as facsimile (fax) machines. In the area of fax machines, the speed of the printer is a limiting factor in that it must be able to print at a faster rate than the telecommunications circuitry can send or receive. If not, either data will be lost, or the fax buffer will fill and the transmission will slow down to the speed of the printer. This results in additional carrier charges due to the additional time taken to complete the transmission. Likewise, print quality is also a concern with fax machines. Therefore, the issues and concerns of applications such as fax transmission are substantially the same as those related to traditional printers.
As improvements have been made in the areas of speed and print density, machines have been developed with increasingly finer and more delicate heating elements which, at the same time, are able to heat up and cool down at increasingly rapid rates. As a result of these advances, high voltage devices such as thermal printers are susceptible to thermal damage in power loss situations. The high voltage print heads are typically controlled by low voltage control logic. When power is lost, both the high and low voltage systems drop at a rate governed by the particular components. Typically, a consequence of power loss is that the low voltage control logic is deactivated first due to the earlier reduction of low voltage power to a level where the control logic fails. After the control logic has failed, the high voltage (while partially reduced) is still at a level where active heating elements can continue to heat. Any elements which were being actively heated at the moment of control logic failure will not be shut off by the control logic. Due to the high heating rates of these devices, the extended heating time allows those heating elements to continue to generate heat until they are damaged.
The prior art has not provided the means to extend the amount of time available to control logic in power down situations to effectively control high voltage devices until the high voltage has dropped to a point where it no longer endangers components.